Method of restricting the oxidation of sponge iron

ABSTRACT

The oxidation of sponge iron, as generally made by the direct reduction of an iron ore, is prevented or limited by circulating through the mass of sponge iron a gas stream containing air and/or nitrogen together with ammonia.

Thumm et al.

1541 METHOD OF RESTRICTING THE 2,156,357 5/1939 simpson 2i/2.5 Ux OXIDATION 0F SPONGE IRON 2,160,831 6/1939 Colby etal ....21 /2.5 UX Q 2,616,780 ll/l952 Atkinson et al..,. 2l/2.5 [75] Inventors' nimflm Thmm Ffkfut al? 2,677,669 5/1954 Amberg ....252/472 ,am' fst age efgen'E 3,111,371 11/1963 Bull 2l/2.5 hem both 0f Germany 3,188,166 6/1965 Dietz et a1. .J1/2.5 [73] Assignees: Metallgesellschaft Aktiengesellschaft, 3551315 l2/1970 Claiborne et al- -7526 UX Reuterwegs Germany; The Steel 3,562,780 2/1971 Eisenberg ..75/26 Company .of camada Ltd" Ham'l' FOREIGN PATENTS 0R APPLiCATIoNs ton, Ontario, Canada g [22] Filed: Aug. 12, 1971 649,735 l/l95l Great Britain 2l/2.5

[2l] Appl` N0.: 171,109 Primary Examiner-Barry S. Richman Attorney-Karl F. Ross [30] Foreign Application Priority Data [57] ABSTRACT Sept. ll, i970 Germany P l9 27 300.8

The oxidation of sponge iron, as generally made by [52] U.S. Cl. ..21/2.5, 75/0.5 R the direct reduction of an iron ore, iS prevented 0r [5 l] Int. Cl. ..C23f 11/02, C23f 9/02 limited by circulating through the mass of sponge iron [58] Field of Search 2l/2.5; 75/26, 0.5 R a gas stream containing air and/or nitrogen together with ammonia.

[56] References Cited 5 Claims, 1 Drawing Figure UNITED STATES PATENTS 1,896,478 2/1933 Burns ..2l/2.5 UX

i ATHSPHEE/C AIE I o2 Anz AQ .76 l HEAITEE l 1s ,lv/l, SPRAY 9 r .'a/ Ik,

l l I i SPa/vae IRON l THEMUCOUPLE 0 e i .77 '/Qu/D NH3 i I \/7 i 1 1 l g i ,"1 d f/ F1 l 2'? LLEJLH l 73 Meg/gwn f..f,2*5 .1 .78 f5 f 16 l M P dcolmi-:N552LTP) 1451 Mar. 27, 1973 PATENTEUI'MRUIQB HEAIT E E THENOCOUPLE CONDE NSE@ Wilhelm Thumm Hors Nagel /NVEN'I URS.

Attorney METHOD OF RESTRICTING THE OXIDATION OF SPONGE IRON CROSS-REFERENCE TO RELATED APPLICATION This application is related to the copending application Ser. No. 39,630, filed 22 May 1970 by us together with others, and entitled METHODS OF AND AP- PARATUS FOR RESTRICTING OXIDATION OF SPONGE IRON; the present application is commonly assigned with the earlier application.

FIELD OF THE INVENTION Our present invention relates to a method of preventing the reoxidation of sponge iron or of restricting such reoxidation and, more particularly, to a method of and an apparatus for limiting the oxidation of sponge iron in bulk.

BACKGROUND OF THE INVENTION Sponge iron is generally produced by the direct reduction of iron oxide, generally in the form of briquettes or pellets, to produce a mass of elemental iron of relatively high purity without a smelting process. The iron has a sponge structure, i.e. is highly porous or cellular, and thus has a high surfacearea/weight or surface-area/volume ratio and thus is susceptible to reoxidation in the presence of atmospheric oxygen and, generally, atmospheric moisture` Various suggestions have been made as to how to avoid reoxidation of the sponge iron, such reoxidation having numerous disadvantages. Firstly, reoxidation may introduce a large proportion of oxygen into the metal structure and require special treatment during steel-making or other refining processes. Secondly, the oxidation process is exothermic and stored bulk sponge iron can show a spontaneous increase in temperature to 80 to 100 C or even to the point of ignition. Such temperature rises are especially disadvantageous when the sponge iron is stored in the hold of a ship because of thermal stress on the hull or otherwise endanger this conveyance.

Consequently, it has been suggested to coat the mass of spoiige iron with an antioxidation or anticorrosion substance such as oil, paraffin or a synthetic resin. These systems have the obvious disadvantage that they apply other impurities to the iron, are expensive and may not provide adequate protection within the cellular body of the mass. Other methods have been suggested as well, but these have only been applied successfully to small quantities of sponge iron, i.e. bags or sacks thereof. Even techniques for storing the sponge iron under a nonoxidizing atmosphere have not proved to be fully satisfactory.

OBJECTS OF THE INVENTION lt is the principal object of the present invention to provide an improved method of limiting the oxidation or reoxidation of sponge iron which is relatively inex` pensive, effective and free from the disadvantages of earlier systems as described above.

lt is another object of the invention to provide a system for reducing or restricting the oxidation of agglomerated sponge iron, preferably in bulk, during storage and transition.

Yet another object of the invention is to provide a method of and an apparatus for limiting oxidation of sponge iron in lump or agglomerated form which are relatively inexpensive and free from the indicated disadvantages.

Still another object of the invention is to provide a system for the purpose described that does not deposit impurities on the iron.

Yet a further object of the invention is to provide a method of restricting the reoxidation of sponge iron which advances the principles set forth in the commonly owned copending application Ser. No. 39,630, filed 22 May 1970.

SUMMARY OF THE INVENTION ln the above-identified application, it has been pointed out that the reoxidation of sponge iron can be limited or restricted surprisingly, by passing a gas stream through the mass, in spite of the fact that the gas may include air or atmospheric oxygen. While one normally would expect that treatment of a sponge-iron mass with an oxygen-containing gas stream would promote oxidation rather than restrict it, the contrary is true apparently because the gas stream prevents static contact between ambient gases and the sponge iron and removes moisture from the vicinity of the interface. We have now found` that the oxidation-limiting effect can be increased by incorporating in the gas stream contacted with the sponge iron between l and g of ammonia per m3 (standard temperature and pressure STP) of the gas stream circulated through the sponge iron.

With respect to the concept of circulating a gas stream through the sponge iron to reduce the tendency toward reoxidation, the aforementioned application provides for the steps of piling the agglomerated or lump sponge iron in an enclosure having a series of gas nozzles in the bottom, and passing through the spongeiron mass, a gas such as air, nitrogen nitrogen-enriched air. The rate at which the gas` is passed through the mass, determines the rise in temperature thereof so that a rate can be selected to give virtually no rise in temperature or a rise only to the highest possible limit tolerable by the conveyance or storage facility and short of combustion. As pointed out in that application, it was found that the oxidation rate and resultant temperature rise are dependent upon the rate of gas flow, and also, that the moisture content of the gas is a major contributing factor to oxidation of the iron mass. A dehumidifier is provided to remove moisture present in the gas prior to its passage through the mass. The dehumidifter may drop the temperature of the gas and thus a heater may be provided to raise the temperature and, of course, increase the dewpoint. It is therefore important in accordance with the principles of that application to reduce the water-vapor partial pressure as low as possible. The aforementioned application also points out that it is advantageous to collect and recycle the gas, in which case a blower is provided in addition to the dehumidifier and heater while a lime filter can be used to remove any acid constituents which may have been picked up by the gas passing through the iron mass. Such acid constituents have a strong oxidizing action.

As noted earlier, the present invention resides in the addition to the gas traversing the vsponge-iron mass of ammonia (NI-I3), preferablyvwhen the sponge iron is a powder, the gas being otherwise constituted as described in the aforementioned application. The use of ammonia eliminates the need for separate neutralization of acid constituents and also is effective to reduce the oxidation of the sponge-iron still further.

The ammonia can be sprayed in a liquid state continuously or discontinuously in constant or variable quantity and vmay be applied to the sponge iron directly, may be mixed with the gas stream traversing the sponge-iron mass in the receptacle therefore,- or may be mixed with the gas elsewhere along the recirculating path. According to the feature of the invention, the ammonia may be supplied to the oxidation-limiting gas stream by spraying it in liquid form directly from above onto the sponge iron. In this case and generally, we prefer to induce a downward flow of the gas (including the ammonia) through the sponge-iron mass.

According to still another feature, the ammonia is supplied such that l to 160 g/m3 (STP) constitutes its concentration in the gaseous oxidation-limiting medium, the concentration preferably being 2 to 50 g/m3 (STP). It has been found that best results are obtained with quantities and flow rates of ammonia-containing gas ranging between l and l m3 (STP) per m2 of the cross-section of the sponge iron, per hour. The quantity is dependent upon the height of the sponge-iron mass and increases with increasing height. The ammoniacontaining gas is totally or partially circulated through the sponge-iron mass to minimize ammonia loss and the water vapor partial pressure is reduced in a condenser or other water-removal device. It is of course possible simply to pass the ammonia-containing gas through the sponge-iron mass without recirculation. A slight warming of the ammonia-containing gas, especially when the latter has been subjected to water removal, is also desirable.

The supply of the ammonia to the gaseous medium or to the sponge iron can be controlled in any convenient manner although we prefer to sense the temperature of the sponge-iron mass or the gases effluent therefrom and to regulate the ammonia supply in accordance therewith. With a temperature increase, the ammonia content is increased and vice versa.

A significant advantage of the present system is that the reoxidation of the sponge iron is reduced even more than has been possible with the system described in the aforementioned application at relatively low cost so that for any given tolerance of reoxidation and any given mass of sponge iron, the ducts, blowers and valves may be dimensioned somewhat smaller.

We have found that at heights of l to 3 m of the sponge-iron pile, the total gas-flow rate should rangel between 5 and 40 rn"i (STP) /m2-hour, preferably 8 to 25 mf/m2hour. With stacks or piles having a height of 3 to 8 meters, the total gas-flow rate should be l0 to 80 ma/mz-hour, preferably to 50 mi/mhour. With stacks of a height in excess of 8 m, the gas-flow rate should be 20 to 160 mi/m2hour, preferably 40 to 100 ms/mz-hour. With sponge-iron pile heights above 8 m, the supply of ammonia-containing gas should be 7 to l5 ms/mz-hour, while heights of 3 to 8 rn call for ammoniacontaining gas supply rates of 3 to l0 malmz-hour, and

pile heights of l to 3 m require from l to 8 ma/mZ-hour (STP) of ammonia-containing gas.

DESCRIPTION OF THE DRAWING The above and other objects, features and advantages of the present invention will become vmore readily apparent from the following description, reference being made to the accompanying drawing in` which the sole FIGURE is a flow diagram of a system embodying the present invention. v

SPECIFIC DESCRIPTION In the drawing, we have shown a vessel 10 adapted to receive a charge 29 of sponge iron above a grate 1l having perforations l2 communicating with a wind box 13. A blower l5, connected by the inlet pipe 14 to the storage vessel 10, induces a downward flow of gas through the vessel and supplies the gas to a condenser 16 in which it is cooled to precipitate moisture and reduce the water-vapor partial pressure. The cool gas is returned at 17 via a heater 18 to the top of the vessel. The top of the vessel 19 and the bottom thereof may be removed for filling and discharging the sponge iron mass respectively. Atmospheric air, nitrogen, or air enriched with nitrogen can be led into the system through a valve 2l and an inlet 20.

Liquid ammonia from a tank 25 is supplied via a valve '24 and a pipe 22 to a spray head 23 above the sponge iron. Thermocouples 27 and 28 are provided to regulate the valve 24 and control the flow of ammonia into the system via a control line 26. When recirculation of the ammonia-containing gas stream is not desired, a valve 31 can be opened to discharge the gas at the discharge side of the blower 15. The spray 30 of ammonia is here shown to be directed onto the top of the sponge iron.

SPECIFIC EXAMPLE A cylindrical vessel having a diameter of 0.5 m and a height of 3 m is filled with 830 kg of fresh sponge iron. The sponge iron is traversed by 25 mS/mz-hour (STP) of atmospheric air at a temperature of 30C which is recirculated through the system. Ammonia is introduced into the recirculated gas stream to a concentration of 50 g per m3 (STP) and the gas flow is adjusted so that about 5 m3 of ammonia-containing gas (STP) passes through the sponge iron per m2 of cross-A section' per hour. With slight modification of the ammonia concentration, it is possible to maintain the temperature of the sponge-iron content at 30C, the gas effluent from the sponge iron being cooled to 20C prior to reheating to remove moisture. There is no noticeable oxidation of the sponge iron with time after the initial slight oxidation over a period of l0 hours. Without the ammonia, oxidation appeared to continue for some time.

We claim:

l. A method of preventing oxidation of sponge iron, comprising the steps of piling and enclosing sponge iron to form a gas-permeable mass thereof and passing a gaseous mixture of ammonia with a gas selected from the group consisting of air, nitrogen-enriched air and nitrogen through said mass at a rate of from substantially l to l5 rn3 (STP) per m2 of cross-section of the mass per hour, the concentration of the ammonia in said gaseous mixture being between l and 160 g/ma (STP), and further comprising the step of reducing the monia is introduced into the gaseous mixture by spraying liquid ammonia onto said sponge-iron mass.v

4. The method defined in claim 1 wherein the gasef ous mixture is recirculated through said mass.

5. The method defined in claim 4, further comprising the step of heating said gaseous mixture prior to passage thereof through said mass.

ggf/Sgn UNITED STATE-S PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,723,059 I Dated 27 March 1973 v InVentor-(S) et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the hea-ding, line v30, for the German applieationv serial number read P 20 4K5 022.0

Signed and sealed this 28th day of August 1973.

(SEAL) i Attest: l l EDWARD M.1Y=LETCHER,JR. RENE'D. TEGTMEYBR Attestng Officer Actlng Commlssloner of Patents 

2. The method defined in claim 1 wherein said concentration is 2 to 50 g per m3 (STP).
 3. The method defined in claim 1 wherein the ammonia is introduced into the gaseous mixture by spraying liquid ammonia onto said sponge-iron mass.
 4. The method defined in claim 1 wherein the gaseous mixture is recirculated through said mass.
 5. The method defined in claim 4, further comprising the step of heating said gaseous mixture prior to passage thereof through said mass. 